Even if your data is exclusively in PCU, it’s still a good idea to enter heavy vehicle proportions, even if you just enter an estimate for the whole junction. This is because some parts of the ARCADY model work with PCU and others use vehicles, so the program always needs to be able to convert between vehicles and PCU. If you do not enter HV percentages, there is risk in particular that queue and delay results may not be accurate.
You can see this effect very easily for yourself as follows. Set up a new file, enter some flows in PCU/hr and run the model. Now enter a HV percentage of, say, 50%, and re-run the model. The RFC will stay the same, but the queue and delay values will change. If using Level of Service outputs, these depend on the delay per vehicle, which will increase. Also note that you can choose whether to display results in terms of PCU or Vehicles, independently of the units used for inputs. If comparing before/after results, be careful to use consistent units.